This invention relates generally to a magnetic resonance imaging (MRI) scanner and, more particularly, to a low-noise subassembly for an MRI scanner.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0) in a z direction, the individual magnetic moments of the nuclear spins in the tissue, in attempting to align with this polarizing field, precess about it at their characteristic Larmor frequency. If the tissue is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins and, after the excitation field B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.y, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The magnets used to produce the polarizing field in MRI scanners include superconductive coil magnets, resistive-coil magnets, and permanent magnets. Known superconductive magnets include liquid-helium cooled and cryocooler-cooled superconductive magnets. Typically, for a helium-cooled magnet, the superconductive coil assembly includes a superconductive main coil which is at least partially immersed in liquid helium contained in a dewar surrounded by a dual shield situated within a vacuum enclosure. In a conventional cryocooler-cooled magnet, the superconductive main coil is surrounded by a thermal shield situated within a vacuum enclosure, and the cryocooler coldhead is externally mounted to the vacuum enclosure with the first stage of the coldhead in thermal contact with the thermal shield and the second stage of the coldhead in thermal contact with the superconductive main coil. Nb--Ti superconductive coils typically operate at a temperature of generally 4.degree. Kelvin, and Nb--Sn superconductive coils typically operate at a temperature of generally 10.degree. Kelvin. The vacuum in the vacuum enclosure must be at very low pressure to prevent unwanted heat transfer which can result in magnet "quenching" (i.e., loss of superconductivity).
Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically comprise a single, tubular-shaped superconductive coil assembly having a bore. The superconductive coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils are typically designed to create a polarizing magnetic field of high uniformity within a spherical imaging volume centered at the bore of the magnet where the object to be imaged is placed.
Open magnets typically employ two spaced-apart toroidally-shaped superconductive coil assemblies with the space between the assemblies allowing for access by medical personnel to perform surgery or other medical procedures during MRI imaging. The patient may be positioned in that space and also in the bore of the two spaced-apart coil assemblies. The open space helps the patient overcome feelings of claustrophobia that may be experienced in a closed magnet design.
The imaging gradient fields are produced by a gradient coil assembly positioned between the main, polarizing field coils and the patient. Because the imaging gradient fields are applied as a series of pulses, the gradient coil assemblies of MRI scanners generate loud noises which many medical patients find objectionable. Active noise control techniques have been used to reduce gradient coil assembly noise, including noise-canceling patient earphones. Known passive noise control techniques include locating the gradient coil assembly in the same vacuum enclosure that contains the superconductive main coils.
As disclosed in co-pending U.S. patent application Ser. No. 08/696,077, filed on Aug. 13, 1996, entitled "Low Noise MRI Scanner", and assigned to the instant assignee, vibration isolation mounts may be used to support the MRI gradient coil assembly in a space between the polarizing field magnet and the patient. This space may also be sealed and evacuated. The vibration isolation mounts inhibit transmission of sound from the gradient coils through the supporting structure, and the vacuum impedes transmission of sound through air to the surrounding structures. While these measures are very effective in reducing the level of noise reaching the patent, further reductions are desired.